Abstract We present the analysis and results of a new VRI photometric and
spectroscopic survey of the central ~
region of the galaxy cluster A3921 (
z=0.094). We detect
the presence of two dominant clumps of galaxies with a mass ratio of
~5: a main cluster centred on the Brightest Cluster Galaxy
(BCG) (A3921-A), and an NW sub-cluster (A3921-B) hosting the second
brightest cluster galaxy. The distorted morphology of the two
sub-clusters suggests that they are interacting, while the velocity
distribution of 104 confirmed cluster members does not reveal strong
signatures of merging. By applying a two-body dynamical formalism to
the two sub-clusters of A3921, and by comparing our optical results
to the X-ray analysis of A3921 based on XMM observations (Belsole
et al. 2005), we conclude that A3921-B is probably tangentially
traversing the main cluster along the SW/NE direction. The two
sub-clusters are observed in the central phase of their merging
process (
0.3 Gyr), with a collision axis nearly perpendicular
to the line of sight. Based on the spectral features of the galaxies
belonging to A3921 we estimate the star formation properties of the
confirmed cluster members. Substantial fractions of both
emission-line (~13%) and post-star-forming objects (so called
k+a's, ~16%) are detected, comparable to those measured at
intermediate redshifts. Our analysis reveals a lack of bright
post-star-forming objects in A3921 with respect to higher redshift
clusters, while the fraction of k+a's increases towards fainter
magnitudes (
). Similar results were obtained in
the Coma cluster by Poggianti et al. (2004), but at still fainter
magnitudes, suggesting that the maximum mass of actively
star-forming galaxies increases with redshift ("downsizing
effect"). The spatial and velocity distributions of k+a galaxies do
not show significant differences to those of the passive population,
and to the whole cluster. Most of these objects show relatively red
colours and moderate Balmer absorption lines, which suggest that
star formation has ceased
1-1.5 Gyr ago. Their presence is
therefore difficult to relate to the on-going merging event. We find
that star-forming galaxies share neither the kinematics nor the
projected distribution of the passive cluster members. Moreover,
most emission-line galaxies are concentrated in A3921-B and in the
region between the two sub-clusters. We therefore suggest that the
ongoing merger may have triggered a star-formation episode in at
least a fraction of the observed emission-line galaxies.